Radiography and fluoroscopy are long well known diagnostic imaging techniques.
In a conventional radiography system, an x-ray source is actuated to direct a divergent area beam of x-rays through a patient. A cassette containing an x-ray sensitive phosphor screen and film is positioned in the x-ray path on the side of the patient opposite the source. Radiation passing through the patient's body is attenuated in varying degrees in accordance with the various types of tissue through which the x-rays pass. The attenuated x-rays from the patient emerge in a pattern, and strike the phosphor screen, which in turn exposes the film. The x-ray film is processed to yield a visible image which can be interpreted by a radiologist as defining internal body structure and/or condition of the patient.
In conventional fluoroscopy, a continuous or rapidly pulsed area beam of x-rays is directed through the patient's body. An image intensifier tube is positioned in the path of the beam opposite the source with respect to the patient. The image intensifier tube receives the emergent radiation pattern from the patient, and converts it to a small, brightened visible image at an output face. Either a mirror or closed circuit television system views the output face and produces a dynamic real time visual image, such as on a CRT, a visual image for interpretation by a radiologist.
More recently, digital radiography and fluoroscopy techniques have been developed. In digital radiography, the source directs x-radiation through a patient's body to a detector in the beam path beyond the patient The detector, by use of appropriate sensor means, responds to incident radiation to produce analog signals representing the sensed radiation image, which signals are converted to digital information and fed to a digital data processing unit. The data processing unit records, and/or processes and enhances the digital data. A display unit responds to the appropriate digital data representing the image to convert the digital information back into analog form and produce a visual display of the patient's internal body structure derived from the acquired image pattern of radiation emergent from the patient's body. The display system can be coupled directly to the digital data processing unit for substantially real time imaging, or can be fed stored digital data from digital storage means such as tapes or discs representing patient images from earlier studies.
Digital radiography includes radiographic techniques in which a thin fan beam of x-ray is used, and other techniques in which a more widely dispersed so-called "area beam" is used. In the former technique, often called "scan (or slit) projection radiography" (SPR) a fan beam of x-ray is directed through a patient's body. The fan is scanned across to the patient, or the patient is movably interposed between the fan beam x-ray source and an array of individual cellular detector segments which are aligned along an arcuate or linear path. Relative movement is effected between the source-detector arrangement and the patient's body, keeping the detector aligned with the beam, such that a large area of the patient's body is scanned by the fan beam of x-rays. Each of the detector segments produces analog signals indicating characteristics of the received x-rays.
These analog signals are digitized and fed to a data processing unit which operates on the data in a predetermined fashion to actuate display apparatus to produce a display image representing the internal structure and/or condition of the patient's body.
In use of the "area" beam, a divergent beam of x-ray is directed through the patient's body toward the input face of an image intensifier tube positioned opposite the patient with respect to the source. The tube output face is viewed by a television camera. The camera video signal is digitized, fed to a data processing unit, and subsequently converted to a tangible representation of the patient's internal body structure or condition
One of the advantages of digital radiography and fluoroscopy is that the digital image information generated from the emergent radiation pattern incident on the detector can be processed, more easily than analog data, in various ways to enhance certain aspects of the image, to make the image more readily intelligible and to display a wider range of anatomical attenuation differences.
An important technique for enhancing a digitally represented image is called "subtraction". There are two types of subtraction techniques, one being "temporal" substraction, the other "energy" subtraction
Temporal, sometimes called "mask mode" subtraction, is a technique that can be used to remove overlying and underlying structures from an image when the object of interest is enhanced by a radiopaque contrast agent, administered intra-arterially or intra-venously. Images are acquired with and without the contrast agent present and the data representing the former image is subtracted from the data representing the latter, substantially cancelling out all but the blood vessels or anatomical regions containing the contrast agent. Temporal subtraction is, theoretically, the optimum way to image the enhancement caused by an administered contrast agent. It "pulls" the affected regions out of an interfering background.
A principle limitation of digital temporal subtraction is the susceptibility to misregistration, or "motion" artifacts caused by patient movement between the acquisition of the images with and without the contrast agent.
Another disadvantage of temporal subtraction is that it requires the use of a contrast material and changes in the contrast caused by the agent must occur rapidly, to minimize the occurrence of motion caused artifacts by reducing the time between the first and second exposure acquisition. Temporal subtraction is also not useful in studies involving rapidly moving organs such as the heart. Also, the administration of contrast agents is contraindicated in some patients.
An alternative to temporal subtraction, which is less susceptible to motion artifacts, is energy subtraction. Whereas temporal subtraction depends on changes in the contrast distribution with time, energy subtraction exploits energy-related differences in attenuation properties of various types of tissue, such as soft tissue and bone.
It is known that different tissues, such as soft tissue (which is mostly water) and bone, exhibit different characteristics in their capabilities to attenuate x-radiation of differing energy levels.
It is also known that the capability of soft tissue to attenuate x-radiation is less dependent on the x-ray's energy level than is the capability of bone to attenuate x-rays. Soft tissue shows less change in attenuation capability with respect to energy than does bone.
This phenomenon enables performance of energy subtraction. In practicing that technique, pulses of x-rays having alternating higher and lower energy levels are directed through the patient's body. When a lower energy pulse is so generated, the detector and associated digital processing unit cooperate to acquire and store a set of digital data representing the image produced in response to the lower energy pulse. A very short time later, when the higher energy pulse is produced, the detector and digital processing unit again similarly cooperate to acquire and store a set of digital information representing the image produced by the higher energy pulse. The values obtained representing the lower energy image are then subtracted from the values representing the higher energy image.
Since the attenuation of the lower energy x-rays by the soft tissue in the body is approximately the same as soft tissue attenuation of the higher energy x-rays, subtraction of the lower energy image data from the higher energy image data approximately cancels out the information describing the configuration of the soft tissue. When this information has been so cancelled, substantially all that remains in the image is the representation of bone. In this manner, the contrast and visibility of the bone is substantially enhanced by energy subtraction.
Energy subtraction has the advantage, relative to temporal subtraction, of being substantially not subject to motion artifacts resulting from the patient's movement between exposures. The time separating the lower and higher energy image acquisitions is quite short, often less than one sixtieth of a second.
Details of energy subtraction techniques in digital radiography and fluoroscopy are set forth in the following technical publications, all of which are hereby incorporated specifically by reference:
Hall, A. L. et al: "Experimental System for Dual Energy Scanned Projection Radiology". Digital Radiography proc. of the SPIE 314: 155-159, 1981; PA1 Summer, F. G. et al: "Abdominal Dual Energy Imaging". Digital Radiography proc. SPIE 314: 172-174, 1981; PA1 Blank, N. et al: "Dual Energy Radiography: a Preliminary Study". Digital Radiography proc. SPIE 314: 181-182, 1981; and PA1 Lehman, L. A. et al: "Generalized Image Combinations in Dual kVp Digital Radiography", Medical Physics 8: 659-667, 1981.
Dual energy subtraction has been accomplished, as noted above, by pulsing an x-ray source in a digital scanning slit device at two kVp's, typically 120 and 80 kVp, and sychronizing the pulses with a rotating filter which hardens the high kVp pulses by filtering out the lower energy x-ray. This results in the patient and x-ray detector sequentially seeing high energy and low energy beams from which the mass per unit area of bone and soft tissue can be solved for.
In energy subtraction, it is desirable that the two energy levels should be widely separated. This is necessary in order to accurately define the masses per unit area of bone and soft tissue.
With a slit scanning device, such as described above, sequentially pulsing the x-ray tube at 120 and 80 kVp is technically difficult and gives rise to very difficult problems in a practical clinical device. The switching frequency has to be on the order of 500 Hz. and insufficient photons (x-ray energy per pulse) results when the highest capacity x-ray tubes are combined with realistically narrow slit widths and scanning times.
In connection with CT (computerized tomography) applications, a two layer energy sensitive detector has been proposed In this proposal, a first calcium fluoride layer is provided for sensing lower level x-ray radiation, and a second downstream sodium iodide layer senses higher energy radiation passing through the first layer. Light caused by radiation in each of the two layers is separately sensed by respective photomultiplier tubes.
It has been proposed to utilize semiconductor materials for sensing nuclear radiation in the nuclear camera environment. See, for example, U.S. Pat. Nos. 4,055,765, 4,055,766 and 4,047,037.
It has also been proposed to utilize rare earth tantalates as x-ray sensitive phosphors. See, for example, Brixner, L. H. et al "On the Structural and Luminescent Properties of the M'TaO4 Rare Earth Tantalates", Journal of the Electrochemical Society, Volume 130, No. 12 (1983).
As mentioned above, this application is a continuation-in-part of U.S. patent application Ser. No. 444,605, filed Nov. 26, 1982 now U.S. Pat. No. 4,626,688, issued Dec. 2, 1986. A European patent application No. 83307157.4 corresponding to the above referenced U.S. parent was published in 1984 as EP 0 115 125.